A modern clinical Computed Tomography (CT) system consists in short of a fan-beam geometry with an X-ray source or tube facing an arc-shaped detector. An acquisition of a large number of X-ray projections at different angles around a patient is performed by rotating the source and the detector continuously over 360 degrees within sub-seconds. Both the attenuated (after the patient) and the unattenuated (before the patient) X-ray intensities are recorded, from which a 3D spatial distribution of the linear attenuation coefficients within the patient is reconstructed, accurately delineating organs and tissues.
The detector is one of the most important components of a CT system. Scintillation detectors, which consist of scintillators coupled to photodiodes, are most frequently used in modern CT systems. In these detectors, an interacting X-ray photon is first converted to scintillation lights in the scintillators. Electron-hole pairs are generated through the absorption of scintillation lights in photodiodes. The energy deposited by the interacting photons over a certain exposure time is integrated to obtain electrical signals output by the photodiodes that are proportional to the total deposited energy. In this way, the electronic noise produced by detector elements in the detector and readout electronics is also integrated into the output signals that are transmitted to the data processing system via analog to digital converting application-specific integrated circuits (ASICs) for image reconstruction.
Components within the energy-integrating detector, such as scintillation detector, are very temperature sensitive, in particular the photodiodes. For instance, if the photodiodes are made of silicon, the dark current from the bulk silicon, which is a major source of the electronic noise, will be doubled for every 8° C. temperature increase. It is, thus, desirable to maintain the energy-integrating detector at a controlled temperature both during its operation and system calibration to avoid image quality issues that may be caused by temperature drifts in the detector components.
Methods and devices for thermal control in a modern CT detector typically employ coolers and/or heaters to provide a constant temperature environment while the detector electronics are turned on continuously [1-4]. A typical operating temperature for an energy-integrating detector is higher than 36° C. [5] with its allowance for variation less than 0.5° C.
Photon-counting detectors that may be used in the next generation X-ray and CT imaging systems work in a totally different way as compared to the energy-integrating detectors. Incident X-ray photons are directly transferred to electrical pulses with pulse amplitudes proportional to the photon energies. These electrical pulses are then fed into the corresponding ASIC channels. Each ASIC channel typically contains a charge sensitive amplifier (CSA), a pulse shaper, a number of pulse-height comparators and counters. After being amplified and shaped, each electrical pulse is compared to a number of programmable thresholds and classified according to its pulse height, and the corresponding counter is incremented.
Compared to the energy-integrating detectors, photon-counting detectors have the following advantages. Firstly, electronic noise that is integrated into the signal by the energy-integrating detectors can be rejected by setting the lowest energy threshold above the noise floor in the photon-counting detectors. Secondly, material decomposition, by which different components in the examined patient can be identified and quantified, is ready to be implemented by using the energy information extracted by the detector [6]. Thirdly, more than two basis materials can be used which benefits decomposition techniques, such as K-edge imaging whereby distribution of contrast agents, e.g., iodine or gadolinium, are quantitatively determined [7]. Last but not least, higher spatial resolution can be achieved by using smaller pixel size. Compared to the typical pixel size of 1 mm2 of current energy-integrating detectors, photon-counting detectors usually use sub-square-millimeter pixel size. For instance, a silicon-strip photon-counting detector can hold a pixel size of 0.2 mm2 [8].
The most promising materials for photon-counting X-ray detectors are cadmium telluride (CdTe), cadmium zinc telluride (CZT) and silicon. CdTe and CZT are employed in several photon-counting spectral CT projects for the high absorption efficiency of high-energy X-rays used in clinical CT. However, these projects are slowly progressing due to several drawbacks of CdTe/CZT. CdTe/CZT have low charge carrier mobility, which causes severe pulse pileup at flux rates ten times lower than those encountered in clinical practice. One way to alleviate this problem is to decrease the pixel size, whereas it leads to increased spectrum distortion as a result of charge sharing and K-escape. Also, CdTe/CZT suffer from charge trapping, which would lead to polarization that causes a rapid drop of the output count rate when the photon flux reaches above a certain level.
In contrast, silicon has higher charge carrier mobility and is free from the problem of polarization. The mature manufacturing process and comparably low cost are also its advantages. But silicon has limitations that CdTe/CZT does not have. Silicon sensors must be very thick to compensate for its low stopping power. Typically, a silicon sensor needs a thickness of several centimeters to absorb most of the incident photons, whereas CdTe/CZT needs only several millimeters. On the other hand, the long attenuation path of silicon also makes it possible to divide the detector into different depth segments that are read out individually. This in turn increases the detection efficiency and makes a silicon-based photon-counting detector possible to properly handle the high fluxes in CT.
However, the employment of detector elements in depth segments also brings problems to the silicon-based photon-counting detector. A large number of ASIC channels have to be employed to process data fed from the detector elements. Each of these ASIC channels typically has a power consumption of several milliwatts [9]. A full photon-counting detector with a total area larger than 200 cm2 can consist of millions of such ASIC channels, which means that the total power consumption of the ASICs is on the level of thousands of watts. Consequently, silicon-based photon-counting detectors impose a challenge for the thermal management system since a lot of heat is generated by the photon-counting detector and has to be transported away, for example by water-cooling or advanced air conditioners, which will be expensive.
The prior art thermal management systems that are committed to maintain a general constant temperature environment for an energy-integrating detector will not be suitable for a photon-counting detectors.